This application claims the priority of German Patent Application Serial No. 198 43 921.0, filed Sep. 24, 1998, the subject matter of which is incorporated herein by reference.
The present invention relates to a method and apparatus for producing composite plastic molded articles having at least one layer of web material (sheets, laminates, fabrics, generally flat materials) and a plastic body, with the composite plastic molded articles being made by injection molding.
Such molded plastic articles are currently used, for example, for articles of furniture, household appliances etc. and in the automobile industry for lining the inside of doors, dashboards, trunk linings, visors etc. The coated areas of the molded plastic articles form hereby the visible surfaces, when mounted, and, for esthetic reasons, carry decors and/or have particular surface characteristics, such as color design, surface texture or tactile properties, such as, for example, xe2x80x9csoft-touchxe2x80x9d, i.e. a certain surface resiliency, oftentimes realized by foamed plastic materials. In the following description, coated materials of this type, including sheets, foamed plastics, decorative materials etc. are denoted in general as web materials. Such web materials react oftentimes very sensitive to deformations and heat so that in an injection molding tool embossment paths of up to 150 mm have been used during back injection of web materials in order to reduce the stress on the web material.
For example, EP-0333198 B1 describes a so-called xe2x80x9cvertical machinexe2x80x9d, having an injection mold with mold halves traveling in vertical position for making a composite molded plastic article. Web material is placed on one mold half of the open injection mold, and subsequently the mold is closed, with plastified plastic mass being introduced into the mold during the closing operation. In this way, damage to the web material is supposed to be prevented, when the plastic mass is introduced.
DE-19531143 C2 proposes a different path while also relating to a vertical machine for injection molding molded plastic articles. Web material is placed into the open injection mold, the mold is closed again, thereby deforming the web material, and plastified plastic mass is introduced during the subsequent opening operation into the opening mold. After introduction of the plastified plastic mass, the mold is closed and the plastic mass is formed through pressing within the mold cavity.
A so-called horizontal machine, in which the mold halves move in horizontal direction is known from DE-OS-2548318. Heated web material is placed between the mold halves and negative pressure is applied on one of the mold halves so that the web material rests upon the inner wall surface of the mold half. After closing the mold, except for a remaining gap, plastified plastic mass is injected into the thus slightly enlarged cavity. Finally, the mold is completely closed to allow the plastic mass to spread in the mold cavity.
Conventional wisdom assumed that greater embossment paths cannot be realized in a horizontal machine because the injected plastic mass cake flows downwardly as a consequence of gravitation and thus will not spread in a desired manner.
It would therefore be desirable and advantageous to provide an improved method for producing composite plastic molded articles, to obviate prior art shortcomings and to realize great embossment paths and yet being applicable in vertical machines as well as horizontal machines.
According to one aspect of the present invention, a web material is first placed between the mold halves of an open injection mold, the mold is closed as the web material deforms, negative pressure is applied on one side of the web material and/or excess pressure is applied on the other side of the web material, so that the web material bears snugly against one mold half, plastified plastic mass is introduced in controlled amounts into the mold cavity of the injection mold, as opening of the injection mold is carried out in a controlled fashion until reaching a predetermined position, the injection mold is fully closed, and subsequently, the finished molded plastic article is withdrawn from the mold.
To support pre-forming, the web material may be heated either across the entire area or locally, before placement into the mold or shortly before closing of the mold halves.
The above-referred notion that great embossment paths cannot be realized in horizontal machines or in vertical machines, when the mold flanks are very steep, is based on the thinking that the plastic mass is quenched on the mold wall during injection molding of the plastic mass so as to form a xe2x80x9ccold skinxe2x80x9d which does not adhere to the mold wall surface and thus slides off the steep mold wall surfaces.
In contrast thereto, the invention is based on the surprising recognition that through injection of plastic mass, the so-called mass cake, to the back of web materials does not result in a formation of a cold skin, but rather results in an adhesion of the plastic mass to the web material, thereby preventing a falling off of the plastic mass. In conjunction with the controlled opening speed of the mold and the respective introduction of the plastic mass at a controlled feed rate, very large mold gaps can be realized in accordance with the invention, even when the mold flanks are very steep or in horizontal machines are involved.
According to another feature of the present invention, the feed rate of the plastic mass and opening and closing movements of the injection mold are so selected in dependence on one another and on the shape of the molded plastic article that the pressure of the plastic mass on the web material is substantially minimized and the plastic mass migrates essentially radially with respect to the point of injection and a swell flow is maintained across the entire injection process.
The point of injection of the plastic mass is implemented via a sprue channel or several sprue channels, depending on the configuration of the molded plastic article.
To further support the swell flow, further plastic mass may be fed during closing of the mold halves.